1. Field of the Invention
The invention pertains to apparatus and methods for detecting neutrons and more particularly to neutron detectors containing multiple detecting elements distributed within a volume of neutron moderating material.
2. Description of Related Art
Neutron detection is used in national security (e.g. protection against nuclear terrorism), scientific research (e.g. neutron scattering for materials research), health physics (e.g. monitoring and control of personnel exposure at nuclear power plants), and other application areas. Neutron detector requirements vary according to the application area and specific intended use and can range from simple counting to detecting the presence of a neutron source and providing information about its identity and location. In general, most neutron detectors do not perform in an optimal way for their intended use and the performance of most neutron detectors is well below that of theoretical limits. An example of this is the neutron detectors used in radiation portal monitors. Ideally, one would want to detect 100% of the neutrons emitted by a neutron source present in the object being scanned (e.g. a vehicle or cargo container) as this would maximize the likelihood of the portal monitor determining that the source was present. For neutron detection, most portal monitors use a neutron detector that consists of one or more 3He proportional counters embedded in a blanket of neutron moderator (e.g. high-density polyethylene, or HDPE). For most current systems, a fast neutron (e.g. energy between 100 keV and 20 MeV) entering the surface of the device orthogonally has a probability of being captured and detected in the 3He counter of between 15 and 20%. Not only would one want to know whether or not a source is present, ideally one would also like to know what type of source it is (e.g. potentially threatening or not), how big it is, where it is, etc.
Substantial effort has been expended over many years in the pursuit of improved neutron detectors and many different detection methods have been investigated. Although current devices are far more sophisticated and have much better performance than their predecessors of several decades ago, it is universally recognized that a need exists for even better detectors that approach the theoretical limits dictated by the underlying physics. An ideal neutron detector would be able to perform a number of functions, including neutron detection with maximal sensitivity or efficiency (i.e. the probability that a neutron entering the detector is successfully detected), high-resolution neutron energy spectroscopy capabilities, and the ability to determine the direction of an incoming neutron. Of equal importance, these functions would not compete with each other; instead, the detector would be able to perform these functions simultaneously, in contrast to many existing devices in which maximizing one capability requires minimizing another.
Techniques for determining neutron direction.
Several disclosures describe fast (e.g. energy>100 keV) neutron detectors with directional sensitivity. Penn in U.S. Pat. No. 6,989,541 and Byrd in U.S. Pat. No. 5,345,084 describe devices based on multiple sensing elements stacked next to each other. The Penn device is capture-gated, meaning that when a fast neutron is detected via proton recoil (hydrogen scattering), it must then be captured in a reaction with 3He in order for it be registered. The Byrd '084 device does not include capture-gating. Both the Penn and Byrd devices can produce an estimate of the radial/azimuthal angle of neutron travel but do not produce a polar angle, precluding an estimate of source location in three-dimensional space. Miller in U.S. Pat. No. 5,880,469 discloses an array of scintillating plastic fibers for fast neutron detection. Directional sensitivity is obtained by physically pointing the device in different directions (the device is elongated); when the device is pointed towards or away from a fast neutron source, it produces a higher neutron count rate than when it is pointed perpendicularly to it. Schulte in U.S. Pat. Nos. 5,036,202 and 5,029,262 discloses a bidirectional fast neutron detector that employs a layer of hydrogenous material located adjacently to multiple layers of thin silicon semiconductor for sensing recoil protons, with a repeated structure of the basic detector (hydrogenous material and silicon semiconductor) used to obtain higher neutron detection efficiency, as the detection efficiency of a single instance of the structure is quite low due to the mean free path of the fast neutrons being large compared to the range of the recoil protons in the device. The Schulte device includes segmentation of the silicon semiconductor into thin slices, enabling the device to determine which direction the proton was traveling across the silicon semiconductor (left-to-right or right-to-left), which is indicative of the direction of travel of the incident neutron. The Schulte device is bidirectional and does not provide a directional estimate in three-dimensional space.
Schulte discloses in U.S. Pat. No. 5,659,177 a thermal neutron detector with directional capability based on Gd foils for thermal neutron capture (leading to electron emission) placed next to segmented silicon semiconductor detectors. Multiple layers are used so that the layer closest to a neutron source will produce a higher neutron count rate than one further away from it, due to the further layer being shielded by the closer layer. Schulte describes how the use of multiple sets of panels pointed in different directions can provide full directional coverage.
Microchannel plates (MCPs) and their derivatives are used for radiation detection, including neutron detection. MCPs have been used for directional neutron detection by employing them as collimators to selectively pass neutrons arriving from a narrow range of angular direction and shield neutrons coming from other directions. (See A. S. Tremsin et al, “Very compact high performance microchannel plate thermal neutron collimators,” IEEE Transactions on Nuclear Science vol. 51, no. 3, pp. 1020-1024, 2004.) Although this approach does yield high confidence in the direction of travel of a neutron, it does so primarily for low energy neutrons (e.g. cold and approximately thermal neutrons) and results in most neutrons coming from a direction other than from a narrow angular range being lost completely to the detector rather than being differentiated on the basis of their direction of travel. As most security-related neutron detection applications require detecting only a small increase in neutron fluence through a detector, maximizing neutron detection efficiency is quite important and a high loss rate of neutrons reaching the detector to fates other than detection is undesirable.
Techniques related to neutron energy spectroscopy.
Kronenberg et al. disclose a neutron spectrometer for spectroscopy of neutrons between 1 and 250 MeV. (US patents include U.S. Pat. Nos. 6,349,124; 6,594,332; 6,614,867; 6,625,243; 6,654,434; 6,654,435; 6,678,343; 6,714,616; 6,717,999; 6,765,978; 6,928,130; and 6,954,512.) These patents are based on the concept of a dodecahedron-shaped device that includes polyethylene for fast neutron-to-proton conversion (via hydrogen scattering), absorbers of different thicknesses for reducing proton energies by specific amounts, and detectors (semiconductors are specified in most patents) for detecting the protons and determining their energies. The different patents represent a variety of different variations on this basic concept.
Fehrenbacher et al. in U.S. Pat. No. 6,423,972 disclose a neutron spectrometer based on semiconductor detectors, with a multiplicity of individual detectors consisting of converter layers coupled to semiconductor active detection layers, with the converter and active layers separated by inactive layers. Converter layers proposed include Li, B, N, and H-bearing materials. A neural network is proposed for processing signals from the individual detectors. Fehrenbacher et al are concerned primarily with the details of the semiconductor detectors and signal analysis and do not propose the use of neutron moderator in their system, either as a means of enhancing neutron detection efficiency or as part of a method of obtaining neutron spectral information.
Buchanan in U.S. Pat. No. 5,204,527 discloses a neutron spectrometer based on lithium tantalate. When a neutron is captured by a 6Li atom in the lithium tantalate, measurement of the total energy deposition in the lithium tantalate enables the energy of the neutron to be determined by subtracting the Q value (4.78 MeV) of the 6Li capture reaction. A neutron energy spectrum is then constructed using the data from a number of neutron events.
Brooks et al provide a useful overview of neutron spectroscopy (spectrometry) in their review paper [F. D. Brooks and H. Klein, “Neutron spectrometry—historical review and present status,” Nuclear Instruments and Methods in Physics Research A 476, pp. 1-11, 2002.] That article summarizes the different types of neutron spectrometers that have been described in the literature.
An article by Toyokawa et al. describes a neutron detector whose design is derived from the Bonner Sphere (Bonner Ball) neutron spectrometer concept [H. Toyokawa et al, “A spherical neutron counter with an extended energy response for dosimetry,” Radiation Protection Dosimetry 70, pp. 365-370, 1997]. In the Toyokawa device, three position-sensitive cylindrical 3He proportional neutron counters are placed at right angles (i.e. one each in the x, y, and z directions) inside a sphere of neutron moderator. This concept improves over the classic Bonner Sphere concept by removing the requirement that a series of different measurements be made, with a different thickness of removable moderating outer shell being used around the sphere for each measurement. Yamaguchi et al describe a very similar device in which lithiated glass scintillating optical fibers are used in lieu of the 3He proportional counters for neutron detection [S. Yamaguchi et al, “Spherical neutron detector for space neutron measurement,” Nuclear Instruments and Methods in Physics Research A 422, pp. 600-605, 1999].
R. A. Craig and M. Bliss disclose a neutron spectrometer consisting of a series of layers of lithiated glass fibers for neutron detection sandwiched between layers of hydrogenous neutron moderator, the various layers of fiber having different response functions to neutrons as a function of neutron energy [R. A. Craig and M. Bliss, “Performance of moderating neutron spectrometers that use scintillating fibers,” Transactions of the American Nuclear Society, 83, pp. 258-259, 2000). In U.S. Pat. No. 6,707,047 they disclose a device based on this concept for measuring the hydrogen content of a material using neutron spectroscopy, with the material being placed within a receptacle formed with a container made of neutron moderator in which the lithiated glass fibers are embedded.
Bubble Technology Industries (BTI) of Chalk River, Ontario, Canada, has produced several devices specifically for neutron spectroscopy. The ROSPEC (Rotating Neutron Spectrometer) device consists of six neutron detectors of various design, such that the response function (as a function of neutron energy) is different for each detector. The six detectors are mounted on a rotating circular platform that rotates while a measurement is being taken in order to eliminate any geometric effects that may skew the data due to a detector having a non-uniform directional response. Each of the six detectors is contained within its own housing that is physically distinct from other detectors and their housing (in other words, there is a gap or separation between adjacent detector housings). BTI also manufactures several other devices for neutron spectroscopy such as liquid scintillator-based devices and 3He proportional counters with enhanced efficiency at higher neutron energies (e.g. fast energy).
Techniques related to determining both neutron energy and direction.
Martoff in U.S. Pat. No. Application 2006/0017000 discloses a gas chamber equipped with a series of anodes and electrodes and in which the gas (e.g. 3He) is reactive with low energy neutrons. The device is intended to produce a series of signals from the anodes and electrodes that indicate the energy and direction (and therefore the momentum) of the neutron reaction products (e.g. 3H and a proton from 3He neutron capture). This information is intended to be used to calculate the momentum of the neutron immediately prior to the neutron capture reaction, thereby indicating its energy and direction.
Techniques related to limiting neutron sensitivity to fast neutrons and determining neutron direction.
August in U.S. Pat. No. 5,078,951 discloses the concept of using fissionable materials that can only undergo fission as the result of fast neutron capture and using them as the neutron-sensitive (neutron-reactive) material in a neutron detector, thereby making the detector sensitive to fast neutrons only. August '951 further discloses the ideas of using an array of such detectors, employing a thermal neutron shield around the device to reduce background noise, and obtaining directional sensitivity using the difference in neutron count rate with spatial location in a large array. The disclosed device does not yield spectroscopic information, only a measurement of fast neutrons.
Techniques related to optimizing detector geometry to maximize neutron detection efficiency.
Polichar et al. in U.S. Pat. No. Application 2005/0224719 disclose the concept of a composite neutron scintillator based on sandwiching multiple layers of neutron scintillator between layers of hydrogenous neutron moderator in order to achieve higher neutron detection efficiency than non-layered systems, such as a 3He tube surrounded by a blanket of neutron moderator. The entire composite scintillator is treated as a single sensing unit; there is no discretization or other segmentation of the composite scintillator into individual sensing units.
The concept of a neutron converter layer sandwiched between two semiconductor detectors has been around for some years. McGregor et al. in US Patent Application 2005/0258372 disclose a device based on a modification of this concept in which grooved surfaces are used to increase the effective area of converter layer per unit area of semiconductor.
Odom et al. in U.S. Pat. No. 6,495,837 disclose a neutron scintillator consisting of alternating cylindrical layers of optically transparent neutron moderator and a scintillating material for use in fast neutron detection. Odom et al also identify that the detector has some directional sensitivity in the sense that neutrons traveling down its length are more likely to be detected than neutrons traveling across it, but not in the sense of being able to estimate neutron or source direction in three-dimensional space. In U.S. Pat. No. 6,566,657, Odom et al further disclose a device that in part builds on '837. The device consists of alternating layers of scintillating material and optically transparent hydrogenous neutron moderator material, with the layers “dimensioned to optimize spectroscopic detection efficiency.” However, it is clear that the device described does not actually produce an estimate of the neutron energy spectrum, but instead is “spectroscopic” only in the sense that it detects neutrons between 0.5 and 14 MeV and does not detect neutrons of lesser energy. In U.S. Pat. No. 6,639,210, Odom et al. disclose a device for borehole/well logging that represents the application of their previous patents to this purpose.
Perkins et al. in U.S. Pat. No. 5,680,423 disclose a method for drawing glass scintillating glass fibers for use in neutron detection. They disclose the concept of dispersing the scintillating fibers throughout a block of neutron moderating material in order to maximize the fast neutron detection efficiency of a device based on such fibers.
Hutchinson et al. disclose a combined neutron/gamma detector with enhanced detection efficiency for homeland security applications. (D. P. Hutchinson, R. K. Richards, A. C. Stephan, “Large Area Combined Neutron/Gamma Detector for Homeland Security,” presented at Detector/Sensor Research and Technology for Homeland and National Security: Chemical, Biological, Nuclear and Radiological Weapons, and Toxic Industrial Chemicals, Sep. 14-16, 2004, Gatlinburg, Tenn.) The proposed device is based on two layers of neutron scintillator screen (each layer is read independently of the other) sandwiched between neutron moderator and plastic scintillator for neutron moderation and gamma detection. Hutchinson et al. demonstrate that using two layers of scintillator screen substantially enhances neutron detection efficiency compared to a device using only one screen (32% versus 18% for Pu spontaneous fission neutrons). Hutchinson et al. do not disclose any directional or spectroscopic neutron capability.
Antech, Inc. sells the Model 2442 Active Well Coincidence Counter (AWCC). (This instrument was formerly known as the JOMAR 51 and was sold by Jomar Systems, Inc.) The Model 2442 is used for measuring the uranium and plutonium content of samples (e.g. waste drums). Geometrically it is a hollow cylinder with thick walls, the interior cavity being used to hold the sample being assayed. The walls comprise polyethylene neutron moderator in which forty two 3He tubes (proportional neutron counters) are distributed, being divided into two rings. This spatial distribution of the 3He tubes is used to achieve high neutron detection efficiency (typically around 31%). Additionally, the system is designed to count coincident neutron detection events in different 3He tubes; these events usually indicate fission events in the sample being measured (many fission events release multiple neutrons). As designed and operated, the Model 2442 does not provide information about neutron direction (neutrons are assumed to originate in the sample being measured, which is located inside the cavity in the device) or neutron energy. Calculations performed at Oak Ridge National Laboratory used a Monte Carlo code for simulating AWCCs; simulations of the Model 2442 are described in the ORNL report, S. A. Pozzi, R. B. Oberer, and L. G. Chiang, “Monte Carlo simulation of measurements with an active well coincidence counter.”
Techniques related to neutron detectors consisting of arrays of sensing elements.
In U.S. Pat. No. Application 2005/0094758, Ronaldson et al. disclose a method of monitoring a sample containing a neutron source for spontaneous fission neutrons, including coincident neutrons, in order to estimate the mass of the neutron source material. The device contemplated by Ronaldson is approximately cylindrical with a hollow interior for receiving a sample. The walls of the device are contemplated to include a layer of hydrogenous neutron moderator in which a series of neutron detectors (e.g. 3He proportional counters) are placed.
Devices based on scintillating fibers are known in the art. Disdier et al in US Patent Application 2005/0161611 disclose a neutron detector consisting of an array of neutron-detecting fibers, each fiber being comprised of a hollow glass tube filled with liquid scintillator. A plurality of such fibers are formed into a device for detecting fast neutrons via proton recoil in the liquid scintillator. Disdier et al suggest the fibers could be used with coded apertures for imaging. Tarabrine in U.S. Pat. No. Application 2004/0227098 discloses a neutron detector consisting of layers of hydrogenous scintillating fibers, with each layer at right angles to the previous layer. The device is intended to detect fast neutrons via proton recoil.
Devices based on semiconductor neutron detectors are also known in the art. Albrecht et al. in U.S. Pat. No. 5,281,822 disclose a fast neutron detector based on a large array of PIN diodes, with neutrons interacting directly in the PIN diodes to produce a signal. The device is able to distinguish between fast neutrons above 8.5 MeV (or other selected energy level) and lower energy neutrons. Seidel et al. in U.S. Pat. No. 5,940,460 disclose an array of semiconductor neutron detectors intended for use in a nuclear reactor for monitoring neutron flux across the range from startup to full-power. In the description and examples given, the semiconductor detectors consist of a semiconductor placed adjacently to a converter layer (e.g. LiF) that produces energetic charged particles as a result of neutron interactions. Seidel et al. disclose that by using multiple types of converter materials that have different characteristic changes in responsiveness to neutron energy, some spectroscopic information can be obtained. Carron et al. in U.S. Pat. No. 5,399,863 disclose a thermal neutron detector consisting of stacked CCD arrays with thin boron slabs between them for producing charged particle neutron reaction products that exit the boron and enter the CCD elements, producing pulses. Carron et al. also disclose that discrimination against gamma rays may be performed by rejecting coincident events in adjacent CCD elements, including adjacent elements between stacked CCD arrays.
In U.S. Pat. No. Application 2006/0023828, McGregor et al. disclose an array of micro neutron detectors, each array comprising one or more of the following: (1) a first gas pocket in contact with a neutron reactive material and a second gas pocket in contact with a different neutron reactive material; (2) a plurality of gas pockets and at least two neutron reactive materials different from one another in contact with the gas; (3) a triad of gas pockets, two of the pockets having neutron reactive materials in contact with the gas and one of the pockets having no neutron reactive material in contact with the gas; and (4) two substrates attached to one another to form a plurality of capillary channels capable of retaining a gas and a neutron reactive material in one of the two substrates oriented into the capillary channels. McGregor et al contemplate a layer of neutron reactive material (e.g. 10B, 6Li, 235U) placed adjacent to a gas in which ionizations are produced when an energetic reaction product from a neutron interaction exits the neutron reactive material and enters the gas, with the motion of the ionizations through the gas producing a current that is sensed by nearby electrodes. The use of different neutron reactive materials provides some ability to estimate neutron energy spectrum, such as by using a material that is preferably sensitive to thermal neutrons in one detector and a different material that is preferably sensitive to fast neutrons in another detector. McGregor et al. also contemplate obtaining a rough estimate of neutron energy by placing different thicknesses of moderator over different sections of a detector. Some directional response is also expected; McGregor et al. state that neutrons incident on the front face of the detector are generally more likely to be detected than neutrons entering from the side due to the thickness of the detector. McGregor et al. contemplate enhancing directional sensitivity by placing selectively chosen collimator holes in the detector. While McGregor et al. focus on the use of the device for reactor core neutron flux mapping, they also disclose that the device may be used for neutron imaging in scientific research and detection of nuclear weapons and nuclear weapons-usable material. In U.S. Pat. No. Application 2006/0043308, McGregor et al. disclose a method for fabricating the neutron detectors disclosed in US Patent Application 2006/0023828, and in U.S. Pat. No. Application 2006/0056573, McGregor et al. disclose methods for using the neutron detectors for monitoring nuclear reactor core neutron flux, as was contemplated as a use of the neutron detectors disclosed in US Patent Application 2006/0023828.
Sarantites et al. disclose a neutron detector consisting of an array of hexagonal sensors placed side-by-side to form a partial sphere for detecting neutrons traveling away from the center of the sphere [D. G. Sarantites et al., “‘Neutron shell’: a high efficiency array of neutron detectors for y-ray spectroscopic studies with Gammasphere,” Nuclear Instruments and Methods in Physics Research A 530, pp. 473-492, 2004].
Techniques related to directional detection of radiation and radiation sources generally and are not specific to neutrons.
Chuiton et al. in U.S. Pat. No. 5,073,715 disclose a directional radiation detector consisting of six radiation sensors located at 90 degrees to one another inside a homogeneous sphere and near its surface. The direction to the source is estimated based on the comparative count rates of the six sensors. Chuiton et al. suggest that the best approach is to have all sensors at the same radial distance from the center of the sphere. McGregor et al. in U.S. Pat. No. 6,806,474 disclose a directional radiation detector consisting of a detector that is shielded from radiation across a range of polar angles and unshielded across another range of polar angles. McGregor et al. propose using a set of movable louvers/collimators to change the shielded and unshielded angles, allowing the device to measure the dependency of incident radiation on the direction of radiation travel. Gottesman et al in U.S. Pat. No. 5,036,546 disclose the concept of using a coded aperture array for radiation imaging. The coded aperture consists of a planar-shaped shield with windows in it through which radiation can enter without being shielded. When used in conjunction with a position-sensitive radiation detector, the detector response pattern can be deconvoluted to produce an image of the radiation source. In U.S. Pat. No. 6,528,797, Benke et al. disclose a system for determining the depth distribution of radioactive material located within a source medium (e.g. soil) using a device that is sensitive to radiation emitted within selected ranges of polar angles relative to a detector axis which is substantially perpendicular to an outer surface of a source medium. In U.S. Pat. No. 6,727,505, Benke et al. disclose another system for determining the depth distribution of radioactive material located within a source medium (e.g. soil), the system consisting of at least one central detector; a plurality of separate satellite detectors adjacent to and surrounding the central detector(s), each of the satellite detectors having a different field of view than the other satellite detectors; a plurality of radiation shields, with a radiation shield situated between each pair of adjacent satellite detectors to substantially block ionizing radiation originating outside the field of view of each satellite radiation detector, in which the signals from the central and satellite detectors represent a spectral footprint of an area and spatial distribution of a radiation source within the area.
Bross et al., in U.S. Pat. No. 6,909,098 disclose radiation scintillators used with wavelength-shifting (WLS) fibers for readout. They mention that nano-sized particles can be used to dope atoms that interact strongly with neutrons into a plastic scintillator to give the scintillator neutron sensitivity. The patent teaches how to combine the scintillator with the WLS fibers and how to analyze the data from the neutron scintillator and another type of scintillator to determine the amount of signal coming from neutrons and the amount from other sources that are not of interest.
Grodzins, et al. in U.S. Pat. App. Pub. 2003/0165211 disclose detecting thermal (low energy) neutrons using a scintillator. The specific method for yielding the claimed directionality is not clearly described, although it does apparently stem from the angle between the neutron capture event location and the line of axis through the photomultiplier tube (PMT).
Objects and Advantages
Objects of the present invention include the following: providing a more efficient neutron detector; providing a neutron detector that makes more efficient use of moderator materials; providing a radiation detector with improved discrimination capabilities; providing a neutron detector capable of obtaining information about the energy spectrum of neutrons it detects; providing a neutron detector capable of determining the isotope(s) from which neutrons have been detected, even when significant shielding is present that modifies the energy spectrum of the neutrons that reach the detector from the source; and providing directional information about neutrons reaching the detector. These and other objects and advantages of the invention will become apparent from consideration of the following specification, read in conjunction with the drawings.